1. Field of the Invention
The present invention relates to an optical element used as a lens, a filter, or a mirror, and more particularly to a diffractive optical element and a refraction optical element, which are made of an optical material having a high refraction index dispersion.
2. Related Background Art
Until now, in refraction optical systems constructed by refraction of light only, glass materials having different dispersion characteristics have been combined with one another to reduce chromatic aberration. For example, in an objective lens of a telescope, a lens having a small dispersion is used as a positive lens, a lens having a large dispersion is used as a negative lens, and these lenses are combined with each other so that chromatic aberration caused on an axis is corrected. Therefore, in such a case, where a structure of lenses and a number of lenses are limited, or a case where the kinds of glass materials to be used are limited, it is very difficult to sufficiently correct the chromatic aberration.
Also, SPIE Vol. 1354, International Lens Design Conference (1990), discloses a method of reducing chromatic aberration by using a diffractive optical element having a diffraction grating on a lens surface or in a part of an optical system. This method uses a physical phenomenon in which a direction in a refraction surface of an optical element, in which chromatic aberration with respect to a light beam with a reference wavelength is caused, becomes opposite to a direction in a diffraction surface thereof. Further, according to such a diffractive optical element, in the case where a cycle of a cyclic structure of the diffraction grating is changed, the same effect as an aspherical lens can be provided. Accordingly, this method is extremely effective in reducing the chromatic aberration.
Now, the diffraction action of a light beam will be described. In general, a light beam incident onto a spherical lens and an aspherical lens, which are used as optical elements of a refraction system, becomes one light beam after being refracted on the spherical and aspherical surfaces of the lenses. In contrast to this, a light beam incident onto a diffractive optical element, which is used as an optical element of a diffraction system, is divided into a plurality of light beams of respective orders by the diffractive action of the element.
Accordingly, in order to make full use of a diffractive optical element in the optical system, it is necessary to concentrate light fluxes of a use wavelength region on a specific order (hereinafter referred to as a design order). When the light fluxes of the use wavelength region are concentrated on the specific order, the intensities of diffraction light beams of other diffraction orders become very low. Therefore, it is not possible to image flare light such that the light beam other than the design order is imaged in a location different from an imaging location of the light beam of the design order.
A grating structure of a diffraction grating, which is determined in advance so as to concentrate light fluxes of a use wavelength region on the specific order, thereby sufficiently improving diffraction efficiency, is disclosed in Japanese Patent Application Laid-Open Nos. 09-127321, 09-127322, 11-044808, and 11-044810. According to these publications, a plurality of optical elements are combined with one another to produce a laminate type optical element, which is constructed so as to have high diffraction efficiency in a wide wavelength region by optimally selecting refraction index dispersions of the respective optical elements and shapes of gratings formed in interfaces of the optical elements. More specifically, a plurality of optical materials are laminated on a substrate, and a relief pattern, a step shape, a kinoform, or the like is formed in at least one of their interfaces, thereby producing a desirable diffractive optical element.
According to these prior patent publications, in order to obtain the structure having the high diffraction efficiency in the wide wavelength region, a material with a relatively low refraction index dispersion and a material with a relatively high refraction index dispersion are combined with each other. More specifically, in Japanese Patent Application Laid-Open No. 09-127321, BMS81 (nd=1.64 and νd=60.1: produced by Ohara Incorporated) is used as the material with the low refraction index dispersion. A plastic optical material PC (nd=1.58 and νd=30.5: produced by Teijin Chemicals Ltd.) is used as the material with the high refraction index dispersion. Similarly, in Japanese Patent Application Laid-Open No. 09-127322, LaL14 (nd=1.698 and νd=55.5: produced by Ohara Incorporated), an acrylic resin (nd=1.49 and νd=57.7), or Cytop (nd=1.34149 and νd=93.8: produced by Asahi glass Co., Ltd.) is used as the material with the low refraction index dispersion. The plastic optical material PC (nd=1.58 and νd=30.5: produced by Teijin Chemicals Ltd.) is used as the material with the high refraction index dispersion. In Japanese Patent Application Laid-Open No. 11-044808 and Japanese Patent Application Laid-Open No. 11-044810, C001 (nd=1.525 and νd=50.8: produced by Dainippon Ink and Chemicals, Incorporated), PMMA (nd=1.4917 and νd=57.4), or BMS81 (nd=1.64 and νd=60.1: produced by Ohara Incorporated) is used as the material with the low refraction index dispersion. The plastic optical material PC (nd=1.58 and νd=30.5: produced by Teijin Chemicals Ltd.), PS (nd=1.5918 and νd=31.1), or the like is used as the material with the high refraction index dispersion.
FIG. 1 is a graph showing Abbe numbers and refraction indexes of materials commercially available as optical materials. In FIG. 1, the ordinate indicates a refraction index nd and the abscissa indicates an Abbe number d. The optical materials described in Japanese Patent Application Laid-Open Nos. 09-127321, 09-127322, 11-044808, and 11-044810 as discussed above, are included in FIG. 1. As is apparent from FIG. 1, the refraction index of a general optical material satisfies nd>−6.667×10−3νd+1.70. It should be noted that a straight line shown in the drawing indicates nd=−6.667×10−3νd+1.70.
According to the structure of the multilayer diffractive optical element, as a difference in refraction index dispersion between the material with the high refraction index dispersion and the material with the low refraction index dispersion increases, the diffraction efficiency of the constructed optical element increases, and the angle of view of the optical element becomes wider. In addition, in order to further improve the diffractive optical element, it is necessary to use a material with a higher refraction index dispersion (small Abbe number). By using such a material, chromatic aberration can be corrected with further accuracy. Of the optical materials of organic polymers as shown in FIG. 1, a material with a minimum Abbe number is polyvinylcarbazole (PVCZ) with an Abbe number of 17.3.
However, in recent years, the optical characteristic requirements of the optical element have become more stringent. Accordingly, in the case where light fluxes of a use wavelength region in the diffractive optical element are concentrated on the design order to improve diffraction efficiency, not only is it required that the diffraction efficiency in a use wavelength region (400 nm to 700 nm) be set to 95% or more by using the material with the high refraction index dispersion and the material with the low refraction index dispersion, but it is also required that an optical characteristic in which a light loss rate at an incident angle of 10° is 3.40% or less. In the case of polyvinylcarbazole (PVCZ) with an Abbe number of 17.3, as described below in Comparative Example 1, the diffraction efficiency in the use wavelength region (400 nm to 700 nm) is 95% or more. However, PVCZ does not satisfy the requirement that the light loss rate at the incident angle of 10° is 3.40% or less. That is, light fluxes of the use wavelength region cannot be concentrated on the specific order to achieve desirable high diffraction efficiency.